1. Field of the Invention
This invention relates, in general, to optical printing and, more specifically, to optical systems for controlling scanned beams in electrophotographic printers and copiers.
2. Description of the Prior Art
Light beam scanning systems are used in various types of hardcopy output devices, such as copiers and printers, to form the image to be produced. Usually, a latent image is first formed on a photosensitive member by modulating the light beam as it scans across the photosensitive member. Then, the latent image is developed with toner and transferred to the hardcopy output medium, such as a sheet of paper or transparent material.
One of the difficulties in scanning a light beam across the photosensitive member is the need to correct for spot size and scan velocity uniformity. Since the photosensitive member is flat, and since the source of the light beam is substantially a fixed point in most scanners, the arc produced by deflecting the beam does not exactly follow the surface of the photosensitive member. Therefore, some type of correction is necessary. This is provided in many prior art scanners by the use of an F.theta. lens located in the light beam.
There are many types of F.theta. lenses used in light beam scanners for size and velocity compensation or correction. Many are compound lenses consisting of more than one glass lens cooperating with each other to provide the desired optical characteristics. One disadvantage of such lenses is the fact that they are rather costly and add appreciably to the overall cost of the light beam scanner. Therefore, it is desirable and an object of this invention to provide an F.theta. lens which can be constructed more economically than prior art F.theta. lenses.
One approach to economy in light beam scanners is to use holographic plates, or holograms, to deflect and diffract the light beam. Several scanners are known which use holograms for the deflecting optics of the scanner. U.S. Pat. No. 4,289,371, issued on Sept. 15, 1981, discloses a holographic deflector for a scanner. The background section of that patent identifies other references to scanners with holographic deflectors. However, from an economy standpoint, a scanner using all holograms instead of a combination of lenses, mirrors, or prisms and some holograms, or holographic optical elements (HOE's), as shown by all the prior art, is very desirable.
Limited use of HOE's in F.theta. lenses has provided some prior art on the subject, such as a published article in APPLIED OPTICS, Vol. 25, No. 5, Mar. 1, 1986, p. 794. The lenses discussed in this publication exhibit parameters which are not optimum for many scanner applications. For more sophisticated applications, the one-element holographic F.theta. lens of the referenced prior art does not adequately fulfill the requirements of long focal length F.theta. lenses. Therefore, it is also desirable to provide a scanner wherein the F.theta. lens is constructed of multiple holograms cooperating with each other to provide the optical parameters necessary for high-precision light beam scanners.